1. Field of the Invention
The invention relates to circuits using very high resistance devices to bypass parasitic leakage currents to prevent the leakage currents from accidentally turning on transistors. In particular, the invention relates to circuits utilizing an open base transistor as a low current bypass device.
2. Description of the Prior Art
In integrated circuits, especially in linear integrated circuits, there are many situations wherein the base of an output transistor is driven by the collector of an opposite type transistor. For example, an NPN output transistor may have its base driven by the collector of a PNP transistor. In this case, when the PNP transistor is gated off no collector current flows therein, and the NPN output transistor is also off. However, the parasitic leakage current flowing out of the collector of the PNP transistor into the base of the NPN output transistor may cause an unacceptably large current to flow in the collector of the NPN transistor when it is supposed to be in the "off" condition. The parasitic leakage current may include several components, all of which increase exponentially with temperature. One of these components may be equal to the collector-base leakage current of the PNP transistor multiplied by the current gain beta. This is the well known I.sub.CEO leakage current from collector to emitter with base open. A more complete description of this leakage current characteristic is found in "Transistor Engineering," by Alvin B. Phillips, McGraw-Hill, 1962. Another component may be a second leakage current (which may be the I.sub.CEO leakage current of an NPN transistor having its collector driving the base of the above-mentioned PNP transistor) flowing into the base of the PNP transistor and multiplied by the beta thereof. The total leakage current flowing into the base of the NPN output transistor will then be multiplied by the beta of the output transistor, causing a unacceptably high collector current to flow when the NPN output transistor is supposed to be off. Because of the exponential increase of such leakage currents with temperature, the problem becomes critical at high temperatures if the leakage current flowing into the base of the NPN output transistor is not bypassed, or shunted to ground. For example, at 150.degree. C, the total parasitic leakage current into the base of the NPN output transistor may be as high as 1 microampere. The beta of the NPN output transistor may be equal to 200, and so the resulting current flowing in the collector of the NPN output transistor, which is supposed to be off under the described conditions, may be as high as 200 microamperes. In most cases this current would be unacceptably large.
The most recent method of bypassing the above-mentioned parasitic leakage current away from the base of the NPN output transistor, thereby keeping its emitter-base junction reverse biased, is to connect a pinched resistor between the base of the NPN output transistor and the ground power supply. A pinched resistor normally consists of an elongated P-type diffused region having a contact at each end, and an N-type region diffused into the surface of the P-type diffused region which "pinches off" conduction along the surface thereof and forces the current through the pinched resistor to flow under the N-type region, thereby substantially increasing the effective resistance of the P-type diffused region. However, pinched resistors have several serious shortcomings for use as low current bypass devices as described hereinbefore. The current through a pinched resistor may decrease by a factor of approximately 2 as the temperature increases from -55.degree. centigrade to +150.degree. centigrade, whereas the parasitic leakage current required to be bypassed therethrough may increase by several orders of magnitude. Thus, the pinched resistor must be considerably lower in value than would be required if the current therein had the same temperature coefficient as the leakage current under discussion. Further, the pinched resistor typically has a breakdown voltage of only approximately 7 volts. This is a serious disadvantage, because in many situations it is desirable that the voltage on the base of the NPN output transistor be higher than 7 volts. In general, the leakage bypass device needs to have the characteristic that it holds the output device (i.e., the NPN output device) off when the driving device (i.e., the PNP transistor) is off, and yet does not rob the base drive from the output device when the driving device is on. The temperature coefficient of the leakage current from the driving device should be matched by the temperature coefficient of the current through the leakage shunt device.
The present invention solves these and other shortcomings of the prior art by providing a minimum sized low current bypass device which sinks the leakage current, and has approximately the same temperature coefficient as the parasitic leakage current, and has a high breakdown voltage, and which approximates a constant current source to a greater degree then a pinched resistor.